West Nile virus

Definition

West Nile virus is a disease spread by mosquitoes. The condition ranges from mild to severe.

Causes, incidence, and risk factors

West Nile virus was first identified in 1937 in Uganda in eastern Africa. It was first discovered in the United States in the summer of 1999 in New York. Since then, the virus has spread throughout the United States.

The West Nile virus is a type of virus known as a flavivirus. Researchers believe West Nile virus is spread when a mosquito bites an infected bird and then bites a person.

Mosquitoes carry the highest amounts of virus in the early fall, which is why the rate of the disease increases in late August to early September. The risk of disease decreases as the weather becomes colder and mosquitoes die off.

Although many people are bitten by mosquitoes that carry West Nile virus, most do not know they've been infected.

Risk factors for developing a more severe form of West Nile virus include:

Conditions that weaken the immune system, such as HIV, organ transplants, and recent chemotherapy

Older or very young age

Pregnancy

West Nile virus may also be spread through blood transfusions and organ transplants. It is possible for an infected mother to spread the virus to her child through breast milk.

Symptoms

Symptoms may occur 1 to 14 days after becoming infected. Mild disease, generally called West Nile fever, may cause some or all of the following symptoms:

More severe forms of disease, which can be life threatening, may be called West Nile encephalitis or West Nile meningitis, depending on what part of the body is affected. The following symptoms can occur, and need prompt attention:

Signs and tests

Signs of West Nile virus infection are similar to those of other viral infections. There may be no specific findings on a physical examination. However, up to half of patients with West Nile virus infection may have a rash.

The most accurate way to diagnose this infection is with a serology test, which checks a blood or CSF sample for antibodies against the virus. More rapid techniques using polymerase chain reaction (PCR) may be used.

Treatment

Because this illness is not caused by bacteria, antibiotics do not help treat West Nile virus infection. Standard hospital care may help decrease the risk of complications in severe illness.

Expectations (prognosis)

In general, the outcome of a mild West Nile virus infection is excellent.

For patients with severe cases of West Nile virus infection, the outlook is more uncertain. West Nile encephalitis or meningitis may lead to brain damage and death. About 1 in 10 patients with brain inflammation do not survive.